Abstract:

The present invention provides a method of manufacturing a nitride
semiconductor capable of improving the crystallinity and the surface
state of the nitride semiconductor crystal formed on top of a
high-temperature AlN buffer layer. An AlN buffer layer is formed on top
of a growth substrate, and then nitride semiconductor crystals are grown
on top of the AlN buffer layer. In a stage of manufacturing the nitride
semiconductor, the crystal of the AlN buffer layer is grown at a high
temperature of 900° C. or higher. In addition, an Al-source
material of the AlN buffer layer is started to be supplied first to a
reaction chamber and continues to be supplied without interruption, and
then a N-source material is supplied intermittently.

Claims:

1. A method of manufacturing a nitride semiconductor in which a nitride
semiconductor crystal is grown on top of an AlN buffer layer, the method
characterized in thatthe AlN buffer layer is formed by starting supply of
an Al-source material at a growth temperature of 900.degree. C. or
higher, and then by supplying a N-source material intermittently and
continuing to supply the Al-source material without interruption.

2. The method of manufacturing a nitride semiconductor according to claim
1 characterized in that the AlN buffer layer is formed to have a film
thickness of 20 Å or smaller.

3. The method of manufacturing a nitride semiconductor according to claim
2 characterized in that the crystal growth of a GaN layer to be formed on
top of the AlN buffer layer is carried out at a growth pressure of 150
Torr or higher and a growth temperature of 900.degree. C. or higher.

4. A nitride semiconductor element in which at least an AlN buffer layer
and a GaN layer are formed sequentially on top of a growth substrate, the
nitride semiconductor element characterized in thatthe AlN buffer layer
is formed to have a film thickness of 20 Å or smaller, and so that,
in a crystal-growth process of a GaN-based semiconductor layer to be
formed on top of the GaN layer, a minimum value of reflectance
oscillation of light from the crystal growth surface is set at 4% or
smaller until a sum of the film thickness starting from the semiconductor
layer next to the growth substrate reaches 1 μm.

5. The nitride semiconductor element according to claim 4 characterized in
that a GaN-based laminate including the GaN-based semiconductor layer is
formed.

6. The method of manufacturing a nitride semiconductor according to claim
1 characterized in that the crystal growth of a GaN layer to be formed on
top of the AlN buffer layer is carried out at a growth pressure of 150
Torr or higher and a growth temperature of 900.degree. C. or higher.

Description:

TECHNICAL FIELD

[0001]The present invention relates to a method of manufacturing a nitride
semiconductor including an AlN buffer layer, and also relates to a
nitride semiconductor element.

BACKGROUND ART

[0002]A lot of development has been made in semiconductor elements made of
gallium nitride compound semiconductors, i.e., group-III-V nitride
semiconductors (hereafter referred to as nitride semiconductors). Some of
the applications of nitride semiconductors are: blue LEDs used as the
light sources for illumination, back light or the like; LEDs used for
multicoloration; and LDs. The manufacturing of nitride semiconductor in a
form of bulk single crystal is difficult. Accordingly, GaN is grown on
top of a substrate of different kinds, such as sapphire and SiC, by
utilizing the MOCVD (metal organic chemical vapor deposition) method. The
sapphire substrate is excellently stable in a high-temperature ammonia
atmosphere in the epitaxial growth process, and is especially used as a
growth substrate.

[0003]The manufacturing of nitride semiconductors by the MOCVD method is
carried out, for example, in the following way. Gas of an organic metal
compound is supplied, as the reaction gas, to the reaction chamber in
which a sapphire substrate is installed as a growth substrate. The
temperature for crystal growth is kept at a high temperature of a range
approximately from 900° C. to 1100° C. The epitaxial layer
of GaN semiconductor crystal is thus grown on top of the sapphire
substrate.

[0004]However, the GaN semiconductor layer that is grown directly on top
of the sapphire substrate by the MOCVD method has a hexagonal pyramid
growth pattern or a hexagonal column growth pattern, so that the surface
of the GaN semiconductor layer has a myriad of irregularities and has an
extremely unfavorable surface morphology. Fabrication of light emitting
elements is extremely difficult by use of a crystalline layer of a
semiconductor that has an extremely unfavorable surface morphology with a
myriad of irregularities formed in its surface, such as above-described
one.

[0005]In a method used for the purpose of solving the above-described
problem, the crystal growth of the nitride semiconductor is preceded by
the growth of an AlN buffer layer on top of a growth substrate.
Specifically, a low-temperature AlN buffer layer with a film thickness of
a range from 100 to 500 Å (angstrom) is formed on top of the growth
substrate at a low growth temperature ranging from 400° C. to
900° C. Since GaN is grown on top of the AlN layer that serves as
the buffer layer, this method has an advantage of improving the
crystallinity and the surface morphology of the GaN semiconductor layer.

[0006]According to the above-described method, however, the buffer layer
has to be grown under strictly limited conditions. In addition, the film
thickness of the buffer layer needs to be strictly set within a very
narrow range from 100 to 500 Å. For these reasons, it is difficult to
achieve a high yield and, at the same time, the improvement in the
crystallinity and the surface morphology of the semiconductor. In short,
the method is of little practical use.

[0007]Accordingly, a proposal has been made, as described in, for example,
Patent Document 1 and Patent Document 2. The proposal is to replace the
low-temperature AlN buffer layer with a low-temperature GaN buffer layer
that is formed on top of a growth substrate at a low growth temperature
ranging from 500° C. to 800° C., and then to grow the
nitride semiconductor crystal on top of the low-temperature GaN buffer
layer.

[0008]Patent Document 1: Japanese Patent No. 3478287

[0009]Patent Document 2: JP-B-8-8217

DISCLOSURE OF THE INVENTION

Problems to be Solved by the Invention

[0010]The improvement in the crystallinity and the other characteristic of
the nitride semiconductor crystal can be expected according to the
above-described conventional technique, but the conventional technique
has the following problems. In the formation of the nitride semiconductor
crystal, which is carried out after the growth of the low-temperature GaN
buffer layer, the growth temperature has to be raised up to a high
temperature of 1000° C. or higher. As the temperature is rising,
the low-temperature GaN buffer layer is being degraded, so that the layer
does not serve as a buffer layer any longer. In addition, the temperature
thus raised up causes another problem: thermal distortion of the GaN
buffer layer that has been formed at a low temperature.

[0011]Furthermore, in both cases of the low-temperature GaN buffer layer
and of the low-temperature AlN buffer layer, a smaller film thickness of
the buffer layer makes it more likely that the crystal axes of the GaN
film, the crystal of which is grown on top of the buffer layer, are
aligned in the same directions, resulting in better crystallinity of the
GaN film. In contrast, as the film thickness becomes smaller, hexagonal
facets become more likely to be formed in the surface, and the surface
morphology of the GaN film is worsened. A problem is brought about by the
use of such a buffer layer in fabricating a device.

[0012]A method has already been proposed to address these problems. In the
method, a high-temperature AlN buffer layer that is formed at a high
temperature of 900° C. or higher is grown on top of a growth
substrate, and then a layer of the nitride semiconductor crystal is
deposited on top of the AlN buffer layer. The high-temperature AlN buffer
layer is, however, is grown under difficult conditions, so that the
crystallinity and the surface morphology of the nitride semiconductor
crystal deposited on the AlN buffer layer are sometimes worsened.
Fabrication of nitride semiconductor crystals of favorable quality is
difficult for this reason.

[0013]An example of the group-III gas and an example of the group-V gas
used in a conventional way of forming a high-temperature AlN buffer layer
are trimethylgallium (TMA) and ammonia (NH3), respectively. These
source-material gases are supplied to the reaction chamber in accordance
with the time chart shown in FIG. 13. To begin with, the supply of TMA is
started (ON) at a time point t0, and then the supply of NH3 is
started (ON) at a time point t1. Once the supply of TMA and that of
NH3 are turned ON, these source-material gases continue to flow
until the formation of the high-temperature AlN buffer layer is finished.

[0014]A higher mole ratio of NH3/TMA in the high-temperature AlN
buffer layer thus formed worsens the flatness of the surface of the
nitride semiconductor crystal grown on top of the AlN buffer layer. FIG.
14, which shows the surface of the GaN crystal grown on top of the AlN
buffer layer with an NH3/TMA mole ratio of 1800, clearly shows that
the surface is rough.

[0015]In contrast, a lower mole ratio of NH3/TMA worsens the
crystallinity of the nitride semiconductor crystal grown on top of the
AlN buffer layer. This worsened state of crystallinity is shown in FIGS.
15 and 16. FIG. 15 shows the state of the surface of the GaN crystal
grown on top of the AlN buffer layer, and FIG. 16 shows the state inside
the GaN crystal. Note that the mole ratio of NH3/TMA is set at 1200.
FIG. 15 clearly shows that the flatness of the surface of the GaN crystal
is improved quite well, but FIG. 16 shows that the crystallinity of the
GaN crystal is worsened by the Al mixed in the GaN crystal.

[0016]In general, as described above, a smaller mole ratio of N-source
material/Al-source material of the supplied reaction gas has a negative
influence on the crystallinity of the nitride semiconductor crystal
formed on top of the AlN buffer layer. In contrast, a larger mole ratio
of N-source material/Al-source material worsens the surface morphology of
the nitride semiconductor crystal.

[0017]The present invention has been made to address the above-described
problems and aims to provide a method of manufacturing a nitride
semiconductor capable of improving the crystallinity and the surface
state of the nitride semiconductor crystal formed on top of a
high-temperature AlN buffer layer.

Means for Solving the Problems

[0018]To accomplish the above-mentioned object, the invention according to
claim 1 provides a method of manufacturing a nitride semiconductor in
which a nitride semiconductor crystal is grown on top of an AlN buffer
layer. The method is characterized in that the AlN buffer layer is formed
by starting supply of an Al-source material at a growth temperature of
900° C. or higher, and then by supplying a N-source material
intermittently and continuing to supply the Al-source material without
interruption.

[0019]The invention according to claim 2 provides the method of
manufacturing a nitride semiconductor according to claim 1 characterized
in that the AlN buffer layer is formed to have a film thickness of 20
Å or smaller.

[0020]The invention according to claim 3 provides the method of
manufacturing a nitride semiconductor according to any one of claims 1
and 2 characterized in that the crystal growth of a GaN layer to be
formed on top of the AlN buffer layer is carried out at a growth pressure
of 150 Torr or higher and a growth temperature of 900° C. or
higher.

[0021]The invention according to claim 4 provides a nitride semiconductor
element in which at least an AlN buffer layer and a GaN layer are formed
sequentially on top of a growth substrate. The nitride semiconductor
element is characterized in that the AlN buffer layer is formed to have a
film thickness of 20 Å or smaller, and so that, in the crystal-growth
process of a GaN-based semiconductor layer to be formed on top of the GaN
layer, the minimum value of the reflectance oscillation of light from the
crystal growth surface is set at 4% or smaller until the sum of the film
thickness starting from the semiconductor layer that is next to the
growth substrate reaches 1 μm

[0022]The invention according to claim 5 provides the nitride
semiconductor element according to claim 4 characterized in that a
GaN-based laminate including the GaN-based semiconductor layer is formed.

EFFECTS OF THE INVENTION

[0023]According to the present invention, after the supply of the Al
(aluminum)-source material of the AlN buffer layer to be formed at a
growth temperature of 900° C. or higher is started, the supply of
the N (nitrogen)-source material is started. The supply of the N
(nitrogen)-source material is carried out intermittently (on-and-off).
Accordingly, while the mole ratio of the N-source material/Al-source
material is set to be a relatively high value so that Al is prevented
from being taken in the crystallinity of the nitride semiconductor
crystal, the nitride semiconductor crystal can be formed with a favorable
surface morphology.

[0024]In addition, the AlN buffer layer is formed at a high temperature of
900° C. or higher. Accordingly, the temperature at the formation
of the AlN buffer layer scarcely differs from the temperature for growing
the nitride semiconductor crystal the layer of which is to be formed on
top of the buffer layer. The growth of the nitride semiconductor crystal
can be started quickly, and the degradation of the AlN buffer layer by
the heating can be prevented. In addition, the thermal distortion of the
AlN buffer layer by the difference in the growth temperature can be
prevented.

BRIEF DESCRIPTION OF THE DRAWINGS

[0025]FIG. 1 is a chart for illustrating a method of forming an AlN buffer
layer in a method of manufacturing a nitride semiconductor according to
the present invention.

[0026]FIG. 2 is a diagram illustrating an overall structure of a nitride
semiconductor including an AlN buffer layer.

[0027]FIG. 3 is a diagram illustrating an example of specific
configurations for the nitride semiconductor crystal of FIG. 2.

[0028]FIG. 4 is a diagram illustrating the surface of the AlN buffer layer
formed according to the manufacturing method of the present invention.

[0029]FIG. 5 is a chart showing experimental data in accordance with the
manufacturing method of the present invention.

[0030]FIG. 6 is a diagram illustrating an example of the configuration for
a nitride semiconductor element including the AlN buffer layer of the
present invention.

[0031]FIG. 7 is a chart showing the relationship between the film
thickness of the AlN buffer later and the crystallinity of a GaN film
formed on top of the AlN buffer layer.

[0032]FIG. 8 is a chart illustrating the voltage-current characteristics
of the nitride semiconductor element of FIG. 6.

[0033]FIG. 9 is a schematic chart of the change in the reflectance of the
surface of the semiconductor layer measured while the crystal of the
semiconductor layer is being grown.

[0034]FIG. 10 is a chart illustrating the reflectance change in the
growing process of the GaN layer in the case where the AlN buffer layer
has a film thickness of 46 Å.

[0035]FIG. 11 is a chart illustrating the reflectance change in the
growing process of the GaN layer in the case where the AlN buffer layer
has a film thickness of 67 Å.

[0036]FIG. 12 is a chart illustrating the reflectance change in the
growing process of the GaN layer in the case where the AlN buffer layer
has a film thickness of 13 Å.

[0037]FIG. 13 is a chart for illustrating a conventional method of forming
an AlN buffer layer.

[0038]FIG. 14 is a diagram illustrating the surface of the AlN buffer
layer formed according to the conventional method.

[0039]FIG. 15 is a diagram illustrating the surface of a GaN layer
deposited on top of the AlN buffer layer formed according to the
conventional method.

[0040]FIG. 16 is a diagram illustrating the state inside the GaN crystal
illustrated in FIG. 14.

[0059]An embodiment of the present invention will be described below with
reference to the drawings. FIG. 1 is a time chart for illustrating
principal steps of a method of manufacturing a nitride semiconductor
according to the present invention. FIG. 2 illustrates a basic structure
of the nitride semiconductor manufactured according to the manufacturing
method of the present invention.

[0060]An AlN buffer layer 2 is formed on top of a sapphire substrate 1
that serves as a growth substrate, and then a nitride semiconductor
crystal 3 is grown on top of the AlN buffer layer 2. This nitride
semiconductor is formed by a known method, such as the MOCVD method. Note
that the nitride semiconductor crystal 3 represents a quaternary mixed
crystal of AlGaInN, what is commonly known as a group III-V nitride
semiconductor, and can be expressed as AlxGayIn.sub.zN
(x+y+z=1, 0≦x≦1, 0≦y≦1, 0≦z≦1).

[0061]Some of the characteristic features of the present invention are:
the high temperature of 900° C. or higher during the crystal
growth of the AlN buffer layer 2; and the intermittent supply of ammonia
(NH3) serving as the N-source material for the AlN buffer layer 2 to
the reaction chamber while the trimethylgallium (TMA) serving as the
Al-source material for the AlN buffer layer 2 has been supplied and
continues to be supplied to the reaction chamber without interruption.

[0062]As described above, FIG. 13 shows the time chart for the
conventional steps of forming the AlN buffer layer. What is common to the
conventional manufacturing method and that of the present invention
includes: the supply of TMA which precedes the supply of NH3 and
which continues without interruption. According to the conventional
method, however, NH3 also continues to flow without interruption
from the supply start time point t1 till the finishing of the AlN buffer
layer formation. This is the point that makes the conventional method
completely different from the present invention.

[0063]In the time chart shown in FIG. 1, the horizontal axis represents
time while the vertical axis represents the ON-OFF status of the supply.
According to the present invention, the supply of TMA is firstly started
(ON) at a time point t0. TMA continues to be supplied without
interruption, as shown in the upper half of FIG. 1, till the formation of
the AlN buffer layer 2 with a predetermined film thickness is completed.
Subsequently, the supply of NH3 is started at a time point t1.
Suppose that the time point t0 is the reference point and that t0=0.
Accordingly, the start of the NH3 supply is delayed from the start
of the TMA supply by a time length of t1.

[0064]After the supply of NH3 continues for a period of W starting
from the time point t1 and ending at a time point t2, the supply of
NH3 is stopped (OFF), and thus TMA becomes the only raw material
that is still supplied. Only TMA is supplied for a period of L starting
from the time point t2 and ending at a time point t3. Then, the supply of
NH3 is resumed (ON) at the time point t3. Subsequently, NH3 is
supplied for a period of W starting from the time point t3 and ending at
a time point t4, and then the supply of NH3 is turned OFF at the
time point t4. After that, only TMA is supplied for a period of L
starting from the time point t4 and ending at a time point t5.

[0065]Likewise, NH3 is supplied for the subsequent period of W
starting from the time point t5 and ending at a time point t6 and for
another period of W starting from a time point t7 and ending at a time
point t8. The supply of NH3 is stopped and only TMA is supplied for
a period of L starting from the time point t6 and ending at the time
point t7. In this way, the supply of NH3 is turned ON only for the
periods of W, and thus the supply of NH3 is carried out
intermittently. Note that the ON-OFF of the NH3 supply is repeated
approximately four times in FIG. 1, but the number of repetitions can be
increased or decreased when necessary.

[0066]As has been described above, the N (nitrogen)-raw material of AlN is
supplied intermittently so that the mole ratio of N-source
material/Al-source material can be set to be a relatively high value.
Thus, Al is prevented from being taken in the crystallinity of the
nitride semiconductor crystal 3, and, in addition, the nitride
semiconductor crystal 3 can be formed with favorable surface morphology.

[0067]Experiments 1 to 7 shown in FIG. 5 differ from one another in the
following variables: the length of time t1 by which the start of the
NH3 supply is delayed from the start of the TMA supply when the time
point t0=0 in FIG. 1; the duration of the period of W and that of the
period of L; the number of repetitions of the ON-OFF cycle for the
NH3 supply; the film thickness of AlN; and the temperature for the
AlN growth. These variables were set appropriately, and Experiments 1 to
7 were carried out under their respective conditions so as to form the
AlN buffer layer 2 on top of the sapphire substrate 1 in each experiment.
Then, a layer of non-doped GaN was formed, as the nitride semiconductor
crystal shown in FIG. 3, on top of the AlN buffer layer 2. The surface
state of this non-doped GaN layer 31 was observed by an AFM or the like.
Thus, a determination was made whether the surface morphology of the
non-doped GaN layer 31 was favorable or not.

[0068]Note that in each of Experiments 1 to 7, the growth pressure was 200
Torr. Hydrogen gas was used as the carrier gas. The flow rate of this
carrier hydrogen gas (H2) was 14 L/min. As FIG. 5 shows, the flow
rate of TMA was 20 cc/min, and the flow rate of NH3 was 500 cc/min.
A value of approximately 2600 is obtained by calculating the NH3/TMA
mole ratio in this case.

[0069]The temperature for growing the AlN buffer layer in the growth
conditions of each of Experiments 1 to 7 was 1000° C. or higher.
The surface morphology for each of the series of Experiments 1 to 6
except for Experiment 2 was favorable. The unfavorable surface morphology
for Experiment 2 was probably caused by the excessively thin AlN-film
thickness of 30.72 Å and by the excessively small number of
repetitions for the supply ON-OFF (i.e., the number of repetitions for
W+L) as the ON-OFF was repeated only twice.

[0070]Each of Experiment 1 and of Experiment 3 to 7 had a favorable
surface morphology of the non-doped GaN layer. FIG. 4 shows an example of
data on the surface of the non-doped GaN layer. The surface morphology of
the non-doped GaN layer was favorable, so that the flatness thereof was
favorable.

[0071]As described above, the NH3/TMA mole ratio for each of the
series of Experiments 1 to 6 is approximately 2600. In each of these
Experiments 1 to 6, favorable surface morphology as shown in FIG. 4 was
obtained except for a case, such as Experiment 2, where the AlN film
thickness is too small and the ON-OFF is repeated only an extremely small
number of times. Even when the crystal growth was carried out with a
higher mole ratio of the AlN layer than the mole ratio of the AlN layer
of FIG. 14 formed according to the conventional method (the mole ratio in
this conventional case was 1800), the surface flatness was more
favorable, to a satisfactory extent, than the surface flatness in the
conventional case.

[0072]Incidentally, Experiment 7 differed significantly from Experiments 1
to 6 in some variables. The NH3/TMA mole ratio, the growth pressure,
and the like were the same. The duration of the period of W was 4.8
seconds, which was the same as in the cases of Experiments 1 to 6.
Meanwhile, the length of time t1 by which the start of the NH3
supply is delayed was 15 seconds. The duration of the time L during which
the supply of NH3 was stopped was 9 seconds. The W+L was repeated
only once. The film thickness of the AlN buffer layer was 13 Å. In
addition, the temperature for growing the AlN buffer layer was
910° C. Also in this case, the surface morphology of the non-doped
GaN layer formed on top of the AlN buffer layer was, as described above,
favorable.

[0073]FIG. 3 shows a specific example of the nitride semiconductor crystal
3 formed on top of the AlN buffer layer 2 of FIG. 2. In FIG. 3, the
reference numerals that are identical to those in FIG. 2 represent the
identical parts. On top of the AlN buffer layer 2, the non-doped GaN
layer 31, a Si-doped n type GaN layer 32, an MQW active layer 33, and a
Mg-doped p type GaN layer 34 are formed one upon another in this order.
The layers ranging from the non-doped GaN layer 31 to the p type GaN
layer 34 are the equivalent to the nitride semiconductor crystal 3 of
FIG. 2. The above-mentioned semiconductor layers are formed by the MOCVD
method. In addition, the MQW active layer 33 has a multiple quantum well
structure including GaN barrier layers and well layers of
In.sub.X1Ga.sub.1-X1N (0<X1).

[0074]A method of manufacturing the nitride semiconductor of FIG. 3 will
be described next. To begin with, the sapphire substrate 1 serving as the
growth substrate is placed in an MOCVD (metal organic chemical vapor
deposition) apparatus, and is subjected to thermal cleaning under a
continuous flow of hydrogen gas and at a temperature raised up to
approximately 1050° C. While the temperature is kept at that
temperature or is lowered down to an appropriate temperature of
900° C. or higher, the high-temperature AlN buffer layer 2 is made
to grow. The conditions under which this high-temperature AlN buffer
layer 2 is grown may be selected appropriately from the experiment data
shown in FIG. 5. As FIG. 1 shows, the high-temperature AlN buffer layer 2
is formed in accordance with at least the following procedure. The
reaction gas serving as the Al-source material (for example, TMA) is
supplied to the reaction chamber in advance and continues to be supplied
thereto without interruption, and then the reaction gas serving as the
N-source material (for example, NH3) is intermittently supplied to
the reaction chamber.

[0075]Subsequently, the growth temperature is set in a range from
1020° C. to 1040° C., and the supply of TMA is stopped.
Then, the non-doped GaN layer 31 is formed by supplying, for example,
trimethylgallium (TMGa) at a flow ratio of 20 μmol/min. After that,
the n type GaN layer 32 is grown by supplying silane (SiH4) serving
as the n type dopant gas. Subsequently, the supply of TMGa and the supply
of silane are stopped. The temperature of the substrate is lowered down
to a temperature between 700° C. and 800° C. in a mixed
atmosphere of ammonia and hydrogen. Then, an InGaN well layer of the MQW
active layer is formed by supplying trimethylindium (TMIn) at a flow rate
of 200 μmol/min and triethylgallium (TEGa) at a flow rate of 20
μmol/min. Then, only the supply of the TMIn is stopped, and thus the
un-doped GaN barrier layer is formed. Thereafter, the formation of the
GaN barrier layer and the InGaN well layer are repeated so as to form the
multiple quantum well structure.

[0076]After the growth of the MQW active layer 33, the growth temperature
is raised up to a temperature in a range from 1020° C. to
1040° C. Then, the p type GaN layer 34 is grown by supplying, for
example, trimethylgallium (TMGa) serving as the Ga-atom source-material
gas, ammonia (NH3) serving as the N-atom source-material gas, and
CP2Mg (bis-cyclopentadienyl magnesium) serving as the doping
material of p type impurity Mg.

[0077]The semiconductor layers can be formed with their respective desired
compositions, with the desired respective conductivity-types, and in
their respective desired thicknesses by supplying necessary gases
together with hydrogen or nitrogen serving as a carrier gas and by
growing the crystals of the layers sequentially at a temperature range
approximately from 700° C. to 1200° C. The necessary gases
include: reaction gases corresponding to the contents of the
semiconductor layers, such as triethylgallium (TEGa), trimethylgallium
(TMG), ammonia (NH3), trimethylaluminum (TMA), and trimethylindium
(TMIn); silane (SiH4) serving as the dopant gas to form an n type
semiconductor layer; and CP2Mg (cyclopentadienyl magnesium) serving
as the dopant gas to form a p type semiconductor layer.

[0078]As described above, when the formation of the AlN buffer layer 2 is
followed by the growth of the non-doped GaN layer 31, the temperature for
growing the AlN buffer layer 2 may preferably be set from 1000° C.
to 1100° C. Since the crystal growth of the non-doped GaN layer 31
can be carried out also at a temperature in a range from 1000° C.
to 1100° C. as the experiment data of FIG. 5 shows, the growth of
non-doped GaN layer 31 can be started quickly with little need of
changing the growth temperature. In addition, the degradation of the AlN
buffer layer 2 by heating can be prevented. Moreover, thermal distortion
of the AlN buffer layer 2 by the difference in the growth temperature can
be prevented.

[0079]Subsequently, a nitride semiconductor element is formed on the basis
of the results of Experiment 7 shown in FIG. 5 in the following way. As
FIG. 6 shows, an AlN buffer layer 12 is formed in a film thickness of 13
Å on top of a sapphire substrate 11 serving as a growth substrate. A
non-doped GaN layer 13 is formed thinly on top of the AlN buffer layer
12. Crystals included in a GaN-based laminate 40 are grown on top of this
non-doped GaN layer 13. The GaN-based laminate 40 formed on top of the
non-doped GaN layer 13 includes: a non-doped GaN layer 14; an n type GaN
layer 15; an MQW active layer 16; and a p type GaN layer 17. In addition,
a transparent electrode 18 is formed on top of the p type GaN layer 17,
and a p electrode 19 is formed on top of the transparent electrode 18. In
the meanwhile, the n type GaN layer 15 is exposed by mesa etching, and an
n electrode 20 is formed on the exposed surface of the n type GaN layer
15.

[0080]A method of manufacturing the nitride semiconductor element of FIG.
6 will be described next. To begin with, the sapphire substrate 11
serving as the growth substrate is placed in an MOCVD (metal organic
chemical vapor deposition) apparatus, and is subjected to thermal
cleaning under a continuous flow of hydrogen gas, at a temperature raised
up to approximately 1000° C. and at a pressure of 55 Torr. Then,
the temperature is lowered down to 990° C., the high-temperature
AlN buffer layer 2 is made to grow at a pressure of 200 Torr. The
conditions under which this high-temperature AlN buffer layer 12 is grown
are based on the date for Experiment 7 shown in FIG. 5. The AlN buffer
layer 12 is formed so as to have a film thickness of 13 Å.

[0081]After the formation of the high-temperature AlN buffer layer 12, the
supply of TMA is stopped. While ammonia is being supplied, the growth
temperature is set at 900° C. or higher (for example, at
930° C.), and the pressure is set at 150 Torr or higher (for
example, at 200 Torr). The non-doped GaN layer 13 is grown in 0.02 μm
by supplying, for example, trimethylgallium (TMGa) at a flow rate of 20
μmol/min. The pressure for growing the crystal of the GaN layer on top
of the high-temperature AlN buffer layer 12 is thus set at 150 Torr or
higher for the purpose of making the seed for growth larger through a
three-dimensional crystal growth. Meanwhile, the growth temperature is
thus set at 900° C. or higher because a too low growth temperature
worsens the crystallinity of GaN.

[0082]Subsequently, the non-doped GaN layer 14 is formed in 2.5 μm at a
growth temperature from 1020° C. to 1040° C. After the
non-doped GaN layer 13 functions to make the seed of the crystal growth
larger three-dimensionally, the non-doped GaN layer 14 serves as the
transition means to a flat film growth (to a two-dimensional growth). The
temperature for growing the non-doped GaN layer 14 is preferably larger
than the temperature for growing the non-doped GaN layer 13 while the
pressure for growing the non-doped GaN layer 14 is preferably smaller
than the pressure for growing the non-doped GaN layer 13.

[0083]Then the semiconductor layers ranging from the n type GaN layer 15
to the p type GaN layer 17 are formed in the same way in which the n type
GaN layer 32 to the p type GaN layer 34 in FIG. 3, so that no description
will be given here. The n type GaN layer 15 is grown in a thickness of
1.5 μm, the MQW active layer 16 is grown in a thickness of 0.1 μm,
and the p type GaN layer 17 is grown in a thickness of 0.2 μm

[0084]After the GaN-based laminate 40 is formed as described above, mesa
etching is carried out to expose a part of the n type GaN layer 15. The n
electrode 20 is formed on the surface thus exposed. On the other hand,
since the top surface of the p type GaN layer 17 is located in the same
direction as the direction in which the light is extracted, the
transparent electrode 18 is formed on the top surface. The p electrode 19
is formed on top of the transparent electrode 18.

[0085]The transparent electrode 18 has a metal multilayer film structure
of Ni/Au/Ti/Al/Ni, and the films in the transparent electrode 18 are
formed in the thicknesses respectively of 30 Å/40 Å/10 Å/160
Å/15 Å, respectively. Each of the p electrode 19 and the n
electrode 20 has an Al/Ni metal multilayer film structure. The films in
each electrode is formed in 3000 Å/500 Å.

[0086]The structure of FIG. 6 is one for a light-emitting diode. FIG. 8
shows the voltage-current characteristics examined while a voltage is
applied to both the p electrode 20 and the n electrode 20 and a current
is made to flow through the light-emitting diode of FIG. 6. In FIG. 8,
the horizontal axis represents the current (If: the unit for the current
is ampere), and the vertical axis represents the voltage (Vf: the unit
for the voltage is volt). In addition, X (solid line) represents the
voltage-current characteristics of the light-emitting diode with the
configuration of FIG. 6, and Y (dot line) represents the characteristics
measured by replacing the AlN buffer layer 12 in the configuration of
FIG. 6 with a low-temperature GaN buffer layer with a film thickness of
approximately 100 Å.

[0087]Comparison of the curve X with the curve Y in FIG. 8 reveals that X
and Y are located at the same position or X is positioned above Y. The
fact that X is positioned above Y means that the current value of Y is
lower than the current value of X when the same voltage is applied. To
put it differently, the fact that X is positioned above Y means that more
leak current takes place. Since the GaN buffer layer is made to serve as
the buffer layer, coarse surface occurs for each layer of the GaN-based
laminate formed above the GaN buffer layer so as to increase the leak
current. It is shown that, when the AlN buffer layer is used, the current
leak becomes smaller, especially in the low-current range (a range from
10-10 A to 10-7 A).

[0088]FIG. 7 shows the examination results of how the crystal of the
non-doped GaN layer 13 formed on top of the AlN buffer layer is changed
by the film thickness of the AlN buffer layer 12 formed on top of the
sapphire substrate 11 shown in FIG. 6, for example. The crystal of the
non-doped GaN layer 13 was grown while the film thickness of the AlN
buffer layer 12 was changed. Then, the surface of the non-doped GaN layer
13 was scanned by an X-ray diffractometer, and the spectrum thus obtained
was analyzed. The crystallinity was determined by measuring the full
width at half maximum of the spectrum. The AlN buffer layer of a 13-Å
film thickness was formed under the growth conditions of Experiment 7
shown in FIG. 5. The AlN buffer layer of a 46-Å film thickness was
formed under the same growth conditions as those of Experiment 7 except
that the growth temperature was set at 930° C., t1 was set at 18
seconds, and W was set at 450 seconds. In addition, the AlN buffer layer
of a 46-Å film thickness was formed under the same growth conditions
as those of Experiment 7 except that the growth temperature was set at
930° C., t1 was set at 24 seconds, W was set at 2.6 seconds, L was
set at 7.2 seconds, and W+L was repeated 8 times.

[0089]The measurement of the full width at half maximum of X-ray
diffraction was carried out for two different directions by changing the
growth direction of the non-doped GaN layer 13. The direction (0001)
represents the c-axis direction, and the direction (10-10) represents the
m-axis direction. These growth directions was accomplished by making
C-plane {0001} be the principal plane for growth of the sapphire
substrate serving as the growth substrate or by making the M-plane
{10-10} be the principal plane for the growth. In addition, the
crystallinity of a comparative example was measured. In the comparative
example, a low-temperature GaN buffer layer of an approximately 100-Å
film thickness was used in place of the AlN buffer layer 12, and the
crystal of the non-doped GaN layer 13 was grown on top of this
low-temperature GaN buffer layer. Both of the values of the crystallinity
in the case of using the low-temperature GaN buffer layer of the
comparative example are favorable because the crystal axes of the
non-doped GaN layer were aligned in the similar directions. The
comparative example, however, had a problem of an increase in the current
leaking as shown by the curve Y in FIG. 8.

[0090]With respect to the examples using the AlN buffer layer, the full
width at half maximum of X-ray diffraction is shown for each of three
different cases: the AlN buffer layer has a 210-Å film thickness; the
AlN buffer layer had a 46-Å film thickness; and the AlN buffer layer
had a 13-Å film thickness. When C-plane, which is a polar plane, was
the principal plane for growth, the highest crystallinity was obtained in
the case where the AlN buffer layer had a 46-Å film thickness. In
contrast, when M-plane, which is a non-polar plane, was the principal
plane for growth, the highest crystallinity was obtained in the case
where the AlN buffer layer had a 13-Å film thickness. In the
meanwhile, when the voltage-current characteristics were measured with a
46-Å AlN buffer layer 12 in the configuration of FIG. 6, the
voltage-current characteristic thus obtained was approximately similar to
that shown by Y in the graph of FIG. 8 employing the low-temperature GaN
buffer layer, though the result is not illustrated in FIG. 8.
Accordingly, it can be estimated that the GaN-based laminate 40 had a
coarse surface, so that the current leaking was not reduced.

[0091]On the other hand, when the AlN buffer layer has a 13-Å film
thickness, the current leaking was reduced in the low-current range as
shown by X in FIG. 8, and thus a favorable device can be fabricated. As
has been described above, as long as the film thickness of the AlN buffer
layer is kept at 20 Å or smaller, the GaN-based semiconductor layer
surface can be formed so as to make the surface not coarse but
mirror-finished (reducing the current leaking), and the crystallinity can
be improved. Small current leaking in the low-current range can bring
about an improvement in the service life of the device and in the
electrostatic breakdown voltage.

[0092]Then, with the configuration of FIG. 6, infrared rays are radiated
on the surfaces of the semiconductor layers that are grown while the
crystals of the AlN buffer layer 12, the non-doped GaN layer 13, and the
GaN-based laminate 40 are being grown. The reflectance of each of the
surfaces was measured. FIGS. 10 to 12 show the reflectance with the
vertical axis and time (second) with the horizontal axis. Specifically,
an optically-transmissive window was formed in the upper portion of the
growth chamber of the MOCVD apparatus. The reflectance was measured by
disposing an infrared LED and an infrared detector in the vicinity of
this window. When the infrared LED was made to emit light with this
configuration, the infrared rays passed through the window and hit the
wafer placed in the growth chamber and in the midst of the crystal
growth. The infrared rays reflected off the wafer were detected by the
infrared detector disposed in the vicinity of the window. Then, the
reflectance was calculated on the basis of the proportion of the amount
of light emitted by the infrared LED to the amount of infrared rays
detected by the infrared detector.

[0093]FIG. 9 shows what the values of the infrared reflectance stand for.
The point A shown in FIG. 9 is the time when the crystal growth of the
GaN-based laminate 40 is started. The reflectance until the growth of the
GaN-based laminate 40 is started is approximately 8 to 9%. Then, the
crystal growth of the first non-doped GaN layer 14 of the GaN-based
laminate 40 is started at the point A. The period B represents the
initial stage of the growth of the non-doped GaN layer 14. As the growth
progresses and the film thickness increases, the reflectance oscillates,
as in the period C, with a cycle equal to half the wavelength λ
used to monitor the reflectance. This is because, in the case of thin
films formed on a substrate, the X-rays are reflected at the interface
between layers of different electron densities (refractive indices);
thus, interference occurs so as to generate, in the reflectance curve, an
oscillation pattern with a cycle of λ/2. In addition, the initial
stage of the growth of the non-doped GaN layer 14 of the GaN-based
laminate 40 (period B) represents the total film thickness from the
semiconductor layer that is next to the sapphire substrate 11 (growth
substrate) to the non-doped GaN layer 14 that is in the process of
crystal growth. To put it differently, the period B represents the sum of
the film thickness of the AlN buffer layer 12, the film thickness of the
non-doped GaN layer 13, and the film thickness of the non-doped GaN layer
14 that is in the process of crystal growth.

[0094]The cycle of the oscillating pattern, such as one observed in the
period C that succeeds the period B, includes information on the film
thickness, while the amplitude of the oscillating pattern includes
information on the coarseness of the surfaces and the interfaces. With a
coarse surface, the reflectance drastically drops down. Accordingly, the
local minimal values, such as H1 and H2, of the oscillation of the
reflectance in the period B were picked up, and the relationship between
the surface state and the film thickness of the AlN buffer layer 12 were
investigated. FIG. 10 shows the relationship in the case where the AlN
buffer layer 12 has a film thickness of 46 Å. FIG. 11 shows the
relationship in the case where the AlN buffer layer 12 has a film
thickness of 67 Å. FIG. 12 shows the relationship in the case where
the AlN buffer layer 12 has a film thickness of 13 Å. The conditions
under which the AlN buffer layers of a 46-Å film thickness and of a
13-Å film thickness were grown are the same as those described above.
The AlN buffer layer of a 67-Å film thickness was grown under the
same conditions as those for the AlN buffer layer of a 46-Å film
thickness except that the period W was set at 3.6 seconds and L was set
at 12 seconds.

[0095]Comparison of these FIGS. 10 to 12 reveals that the reflectance
during the period B representing the initial stage of growth of the
non-doped GaN layer 14 oscillates significantly in FIGS. 10 and 11 and
reaches approximately 20%. The local minimal value during the period B
reaches approximately 10% in each of FIGS. 10 and 11. In contrast, the
oscillation of the reflectance in FIG. 12 is quite small from the time
point of staring the growth, and the minimal value of the reflectance
oscillations is 4% or smaller during the period B. As has been described
thus far, forming a mirror-finished surface of the GaN layer that is
grown on top of the AlN buffer layer by setting the film thickness of the
AlN buffer layer at 20 Å or smaller is equivalent to making the
minimum value of the reflectance oscillation of the light from the
crystal-growth surface be 4% or smaller at the initial stage of growth
for the GaN-based semiconductor layer crystal-grown on top of the GaN
layer, that is, until the sum of the film thicknesses of the
semiconductor layers starting from the growth substrate reaches 1 μm.
A favorable device can be obtained by fabricating a nitride semiconductor
element in this way.